The need to store digital files, documents, pictures, images and other data continues to increase rapidly. In connection with the electronic storage of data, systems incorporating one or more data storage controllers have been devised. Storage controllers receive data read and write requests from host computers and control one or more physical storage devices to beneficially store or provide the requested data from/to the host computers.
Storage controllers generally buffer read and write data requests, often converting the host data read and write requests into RAID or storage device read or write requests. Many storage controllers store read and write data in cache memories included as part of the storage controllers. Cache memories are small compared to external storage devices such as hard drives, and generally orders of magnitude faster. However, cache memory costs significantly more per byte than storage devices, and therefore cache memory size is correspondingly small in order to be cost effective. The need is always present for cache memories to operate as efficiently as possible in order for overall storage controller performance to be maximized to all interconnected host computers.
Many storage controllers have separate areas of memory dedicated to read cache and write cache. If requested data is in the read cache when a host computer requests the data that is a “cache hit”. If requested data is not in the read cache when a host computer requests the data that is a “cache miss”. In the case of a read cache, it is advantageous for a cache memory to store data before a host computer requests the data. In this way, the requested data can be provided at faster cache memory speeds instead of slower storage device speeds. In order for a read cache to contain data before a host computer requests the data, it is necessary for the storage controller to predict in advance which data is likely to be requested, and perform a read ahead operation to read the data from storage devices and store the data in the read cache. Read ahead operations are desirable since they improve the ratio of cache hits to cache misses.